1. Field of the Invention
The present invention relates to a smart antenna receiving system, and more particularly, to a normalizing apparatus for adaptive beamforming in a smart antenna receiving system.
2. Description of the Related Art
When processing an individual user signal in a mobile communications system, such as a code division multiple access (CDMA) system, other user signals are considered as interfering signals. Therefore, as the number of users sharing the same frequency at a base station increases, multiple access interfering noise increases and a bit error rate of a received signal increases, thus resulting in performance deterioration of the CDMA system. In order to increase the capacity and communication quality of the CDMA system, it is essential to remove inter chip interference due to multi-path fading of an individual user signal, and multiple access interference due to multiple user signals. In general, inter chip interference can be removed using a RAKE receiver capable of separating two signals with different time delays, and multiple access interference can be removed using a smart antenna receiving system.
Smart antenna technology has been developed to cope with the limited frequency resources in a mobile communications system and the explosive increase in application for mobile users. In detail, smart antenna technology adjusts the gains and phases of signals received by antenna, using a plurality of antenna arrays, so that a base station can receive only a signal transmitted from the direction a user desires. This remarkably reduces the level of noise signals due to multiple access interference, which is caused by the signals transmitted from other directions. In this way, the performance of the system and the channel capacity of the base station can be increased.
Depending on beamforming method, the smart antenna receiving system is largely categorized into two types: a switched beam smart antenna with a fixed antenna pattern; and an adaptive beam smart antenna with a variable antenna pattern according to temperature or ambient conditions. The former one has the disadvantage of deteriorating system when a user is positioned between antenna patterns, whereas the latter one is capable of forming a beam directly toward a user. Therefore, the latter one is less affected than the former one by change in the ambient conditions.
An adaptive beamforming algorithm, which is an important technology in the smart antenna receiving system, enables estimation of a signal to be transmitted by multiplying signals received via antenna array by proper weight vectors. In particular, a normalized least mean square (NLMS) algorithm is often used to compute a weight vector. However, if a normalizing apparatus, which performs operations using the NLMS algorithm, is realized by hardware, the size of required multiplication and division operation units might be ten times more than that of a receiver. Even if this significant figure is reduced and timing is used after dividing, the size of the normalizing apparatus would still be equivalent to that of the receiver. In particular, a mobile communications system that processes signals in real-time requires a simplified NLMS. However, the inclusion of the multiplication and division operation units in the normalizing apparatus makes it almost impossible to simplify the structure of the normalizing apparatus. In addition, since clocks more than bits of dividend are required for the division operation unit, the size of the normalizing apparatus is increased and real-time processing velocity decreases due to a bottleneck phenomenon.